The present invention relates to a method for accomplishing a handover in a cellular communication system which comprises at least one base station per each cell, which base station is controlled by a base station controller which controls one or more base stations, and which base station controller with the base stations under its control forms a base station system.
The present invention is particularly applicable to CDMA cellular communication systems. A CDMA (Code Division Multiple Access) system is a multiple access method which is based on spread spectrum technology and whose application in cellular communication systems has lately been initiated along with the earlier initiated FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access) technologies. The CDMA technology has several advantages over the earlier initiated methods, such as spectral efficiency, simple frequency planning and improved traffic capacity.
In a CDMA method, the narrow-band data signal of the user is multiplied by a spreading code of much wider bandwidth, to a relatively wide traffic channel band. In the known experimental cellular network systems, the bandwidths used on traffic channels include, for example, 1.25 MHz, 10 MHz and 25 MHz. In the multiplying process, the data signal spreads to the whole band used. All users transmit simultaneously by using the same frequency band, i.e. traffic channel. A separate spreading code is employed for each connection between a base station and a subscriber terminal equipment, and the signals from the users can be identified from each other in the receivers on the basis of the spreading code of each connection.
In a CDMA system, then, all users transmit on the same, relatively wide frequency band. The traffic channel of the user is formed by a spreading code which is characteristic to the connection and on the basis of which the transmission of the user is identified from the transmissions of other connections, as described earlier. As a considerable number of spreading codes are usually in use, the CDMA system does not have a definite capacity limit like the FDMA and the TDMA systems. The CDMA system is a so-called interference restricted system, in which the number of users is restricted by the level of interference allowed for one user to cause to another. As the spreading codes of the users in a given cell in the systems in use are not fully uncorrelated with regard to spreading codes employed by users in neighbouring cells, especially, simultaneous users cause interference to each other to some extent. This kind of interference caused by one user to another is referred to as multiuser interference. As the number of users increases, the level of interference they cause to each other consequently increases, and, as the number of users reaches a certain level, the interference increases so as to damage the quality of the connections. In a CDMA system, it is possible to determine an interference level not to be exceeded, and so to set a limit to the number of simultaneous users, i.e. limit to the capacity of the system. A temporary exceeding of this number can, however, be allowed, which means that some of the connection quality is sacrificed at the expense of capacity.
In a typical mobile station environment, signals between a base station and a mobile station travel by several different paths between the transmitter and the receiver. This multipath propagation is mainly caused by signals reflecting from surrounding surfaces. Signals that have travelled from a transmitting user via different paths arrive at a receiver at different times due to different delays in the propagation time. The CDMA method differs from the conventional FDMA and TDMA methods in that in the CDMA method, the multipath propagation can be utilized in the reception of signals. As a CDMA receiver, a so-called RAKE receiver consisting of one or more RAKE branches is commonly employed. Each branch is an independent receiving unit whose function is to compose and demodulate one received signal component. Each RAKE branch can be controlled to synchronize into a signal component that has travelled along a path of its own, and, in a conventional CDMA receiver, the signals of the receiving branches are advantageously combined, and thus, a good-quality signal is obtained.
It is possible that the signal components received by the branches of a CDMA mobile station receiver have been transmitted by one or more base stations. In the latter case, a so-called macro-diversity is involved, i.e. a diversity mode by which the quality of a connection between a mobile station and a base station can be improved. In CDMA cellular communication networks, macro-diversity, also referred to by the term soft handover, is employed for ensuring the operation of power control at the base station border areas, and for enabling a seamless handover. Thus, a mobile station employing macro-diversity simultaneously communicates with two or more base stations. All connections transmit the same information. As an example of a cellular communication system employing macro-diversity, the publication EIA/TIA Interim Standard: Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, TAI/EIA/IS-95, July 1993 is referred to.
Thus, in a macro-diversity situation, the terminal equipment can combine signals transmitted by different base stations. At the base station end, signals received by two separate base stations from the terminal equipment are combined at the first possible point, which in most cases is the base station controller in whose area the base stations are located. If the base stations to which the terminal equipment is coupled are under control of different base station controllers, the practical implementation of a soft handover will be considerably complicated, because in such a case, the connecting has to be carried out in a mobile services switching center.
Older cellular communication systems, such as GSM, NMT and AMPS, employ a so-called hard handover, in which the base station change is carried out by first breaking the connection to the old base station and then establishing connection to a new base station. In such a case, then, the terminal equipment is coupled to only one base station at a time. Hard handover technology is simpler to implement than is soft handover technology. So far, hard handover has not been applied to CDMA systems, because it causes instability in power control. Because the system is interference restricted, accurate power control is a prerequisite for the operation of a CDMA system.